Secondary batteries called lithium ion batteries, currently attract much attention. Those batteries can be roughly classified into two categories of so called lithium ion secondary batteries and lithium metal secondary batteries. The lithium metal secondary batteries utilize precipitation and dissolution of lithium for the operation. On the other hand, the lithium ion secondary batteries utilize storage and release of lithium in the charge-discharge reaction. These batteries each can realize charge-discharge at large energy densities, as compared to lead batteries or nickel-cadmium batteries. By making use of those characteristics, in recent years, these batteries have been widely applied to portable electronic equipment, such as camera-integrated VTR's (video tape recorders), mobile phones, and notebook computers. In accordance with a further expansion of applications, the development of lightweight secondary batteries, which can allow higher energy densities, has been advanced, as power sources of the portable electronic equipment. Furthermore, there is a strong demand for size reduction, service life prolongation, and safety enhancement.
Regarding a liquid electrolyte, a particular combination of materials has widely been employed, for lithium ion secondary batteries or lithium metal secondary batteries (hereinafter, these may be collectively referred to simply as a lithium ion secondary battery). That is, a carbonic acid ester-based solvent, such as propylene carbonate or diethyl carbonate, is employed, in combination with an electrolyte salt, such as lithium hexafluorophosphate. This is because these compounds have a high electric conductivity and also these are potentially stable.
On the other hand, the conventional liquid electrolyte contains the aforementioned low-molecular weight organic compounds as the components. Thus, it is important to impart flame retardancy to the liquid electrolyte, from the viewpoint of safety. For the purpose of achieving improvement in this respect, a technique has been proposed to add a cyclic phosphazene or a phosphate compound into the liquid electrolyte (see Patent Literatures 1 to 3).